The present invention relates to a method and an apparatus for continuous hardening of light-curing resins, and in particular to a method and an apparatus for continuous hardening of light-curing resins by continuously hardening the objects made of light-curing resins while they are in movement.
The method and the apparatus for hardening the light-curing resins are divided into those using ultraviolet light and those using visible light.
The hardening apparatus using ultraviolet light is mostly used for the hardening of the thinner objects with thickness of several .mu.m to several hundreds of .mu.m such as film, ink, coating film, etc. of light-curing resins.
These apparatuses comprise the continuous moving means such as belt conveyor, and the object thinly coated with light-curing resins are continuously hardened by irradiating ultraviolet light. The apparatuses have high productivity and are widely used.
On the other hand, the method and the apparatus using visible light are applied, for example, for the hardening of dental laboratory products using composite resin in the field of dentistry. For example, the Japanese Provisional Patent Publication Sho 62-38149 describes an irradiation apparatus having fluorescent lamp to emit the visible ray of 400-500 nm, and the Japanese Provisional Patent Publication Sho 62-47354 discloses an irradiation apparatus to irradiate the light from halogen lamps on the objects on a turn-table.
The hardening apparatus as described above is designed in batch type, which normally hardens one or more objects at one time.
However, when ultraviolet light is used as described above, high productivity is obtained, whereas the object cannot be hardened to its deep core. In case of dental material with thickness of 1 mm or more, the interior of the object often remains unhardened, and the hardness and the mechanical strength of the products are reduced.
To solve these problems, photopolymerization type resin is coated thinly, and ultraviolet light is irradiated for hardening. Then the light-curing resins is thinly applied again, and ultraviolet light is irradiated again for hardening, repeating the procedure by many times. This requires a complicated procedure and much time.
Because a normal ultraviolet light irradiation apparatus has constant radiation flux density, the rate of polymerization cannot be controlled. If the radiation flux density is increased in order to harden thick light-curing resins, the objects are rapidly polymerized and hardened, and internal distortion occurs inside the hardened object during polymerization, and this results in the decrease of mechanical strength or the reduction in dimensional accuracy. On the other hand, by the hardening apparatus using visible light, it is possible to harden easily the object with thickness of 1 mm or more, whereas only the conventional batch type apparatus is available, and the productivity is low.
Further, the illuminance is fixed in the conventional type visible light hardening apparatus. If the illuminance is increased to achieve the hardening to the deep core of the object, the rate of polymerization cannot be controlled as in case of ultraviolet light. As the result, internal distortion occurs inside the hardened object during polymerization, and this results in the decline of mechanical strength and lower dimensional accuracy.
Also, in the apparatus using ultraviolet light or visible light, when the flux density of the irradiation is weak, light energy converted to heat is low. Thus, the atmosphere temperature is not increased sufficiently, and this leads to lower hardness and poor mechanical strength of the hardened object. On the other hand, if the flux density of the irradiation light is strong, the energy to be converted to heat also increases. As the result, the hardened object is heated more than actually required, causing thermal expansion. This results in the lower dimensional accuracy of the products when the hardening is completed.